Blade root extenders are used to increase the rotor diameter of a wind turbine generator in order to use an existing blade on turbines situated in locations where the wind class is lower than the basic design wind class for the blade (and wind turbine). The increased rotor diameter increases the blade sweep area and consequently increases the wind turbine's electric power production.
The blade extenders are generally made out of metal in a welded or cast design. Such extenders have two bolt circles: one for blade connection and the other for bearing (or hub) connection, therefore including two screwed flanges and one cylinder between them that works as a separator.
In principle, this type of extender can also be manufactured in composite, such as plastic reinforced with fibreglass or reinforced with carbon fibre, as indicated in WO0142647, showing the design of an extender that increases the blade length and which, like the blade, is made out of composite.
Further using an extender could facilitate a blade angle change, like in WO03060319 where the angle between the tower axis and the rotor circular area is modified to avoid collision between blade tip and tower in case of strong winds. The blade angle in circumferential direction could also be changed as seen in FR2863318.
Several wind turbine manufacturers have suggested or used a design with long stud bolts with two concentric cylinders made out of longitudinally welded steel plate. The bolts are in the one end connected to the blades and in the other end is clamping the bearing inner ring.
However, given that the wind turbine blades, and therefore the extenders, are subjected to severe fatigue loads, they should be correctly designed so that the use of long prestressed bolts ensures that the extender is never subjected to traction loads, considerably increasing the fatigue resistance of the extender. The compression load on the cylinders leads however to the presence of tensile hoop stresses in the cylinder, which must be considered.
The blade root extender does however not contain a circumferential weld for the assembly of the extender. The circumferential weld is typically the weakest point in the design when considering fatigue.
On the other hand, due to the compression preload from the bolts the wall thickness of the cylinder is defined by either the buckling strength of the cylinders or the compressive strength of the material.